1. Field of the Invention
This invention relates to methods of manufacturing metal and ceramic matrix composite materials and, more particularly, to methods of manufacturing metal and ceramic composites utilizing thermal spray techniques and an optional finishing step of diffusion annealing and/or hot isostatic pressing.
2. Description of the Related Art
Components for the aircraft and aerospace industries require materials having maximum specific strength and specific modulus. Specific strength is the ratio of tensile strength to density, and specific modulus is the ratio of modulus of elasticity to density. These quantities present the structural properties in terms of what used to be called the strength-to-weight ratio.
Composite structures offer significant weight economy to the engineer when used in structural designs. A composite structure consists of a continuous-phase matrix material which is made stronger and/or stiffer by a second material having a substantially higher tensile strength and/or modulus of elasticity. The material used for reinforcing the matrix can be in the form of fibers, woven textiles, or particles.
A simplified version of the theory behind the reinforcement effect of adding the high-strength/stiffness second-phase material is that the major portion of an applied load is borne by the second-phase material, while the matrix material serves to maintain the geometric and alignment relationships of individual second-phase reinforcing material elements with respect to each other for the case of continuous reinforcement. The matrix provides some degree of ductility and toughness to the composite body by transferring and distributing strain in local areas of the continuous reinforcing phase more widely to other second-phase elements and generally acts as a "glue" to hold the composite assembly together as well as to provide a feasible method of manufacturing a specific shape. The direct utilization of the reinforcing phase as a monolithic body is generally not possible because of extreme brittleness or because of difficulty or expense in obtaining it as a monolithic body. The desired strength/stiffness property of the reinforcing phase is only found in the form of structural units having very small dimensions, generally less than 0.010 inch in the smallest dimension.
In another composite form, the reinforcing phase material may be present in discrete form such as relatively short fibers of glass or silicon carbide whiskers, or short lengths of glass, carbon, graphite, partially crystallized carbon, boron or silicon carbide. These composites depend for their strength upon a degree of particle hardening. Such materials include "cermets," which are a mixture of metal and ceramic substances generally compounded with the object of producing a combination of hardness and toughness such as would be required in a tool material. Another related group of composite materials relies upon dispersion hardening, in which the movement of microscopic dislocations is impeded by strong particles having microscopic dimensions also.
If a composite material contains discrete reinforcing elements, these suffer elastic strains when the material is stressed. These elements contribute in this way to the load-carrying capacity of the material and provide obstacles to the movement of dislocations, assuming that the elements themselves are strong. If the volume of such strong elements in the composite is proportionately large, they will provide a high strength and a corresponding high load carrying capacity. One of the best ways of increasing tensile strength is by using elements in the form of long continuous fibers. The matrix material may begin to flow when stressed but in doing so will cause a force to be set up at the surface of the fiber. If the fiber is sufficiently long, the transmitted force will finally lead to its fracture and the fiber will have fully contributed to the strength of the composite material. Obviously the strength will have a maximum value parallel to the direction of the fibers.
The nature of the interface between the discrete elements and the matrix influences the extent to which the load is transferred from the matrix to the reinforcing material. Cohesion at the interface may be achieved by one of several methods:
(1) Mechanical bonding; this involves a large enough coefficient of friction acting between the surfaces.
(2) Physical bonding, which depends upon van der Waals forces acting between surface molecules.
(3) Chemical reaction bonding at the interface; this, however, may give rise to weak, brittle compounds in some cases.
(4) Bonds formed by solid-solution and diffusion effects.
Organic thermoplastic and thermosetting resin matrix composites have been in use for a long time and their fabrication methods are fully described in the technical literature. Structural metal matrix composites are relatively new and thus far only aluminum and, to a lesser extent, magnesium and copper have achieved reasonable degrees of development. Composites of these metals are obtained through powder metallurgy, liquid metal infiltration, and the diffusion bonding of alternate layers of metal foils and filaments. Ceramic matrix composites are most commonly fabricated by cold press and sinter, cast and sinter, or hot press techniques. All of the above fabrication methods suffer in varying degrees from one or more of the following problems: the presence of internal defects such as voids and incomplete diffusion bonds; the breakup of continuous filaments due to the measurable deformation of the matrix in pressing type operations; excessive reaction between the matrix and the reinforcing phase material; low bond strength between the matrix and the reinforcing phase; and very high cost.
Some examples of the art related to the fabrication of composite materials are given below.
U.S. Pat. No. 3,615,277 to Kreider et al is directed to a process of fabricating a multilayer fiber-reinforced metal matrix composite by winding a filament on a spring-loaded mandrel covered with brazing foil, preheating the mandrel, plasma arc spraying metal matrix material in coalescent form onto the filament windings so as to form a monolayer tape, and low-pressure braze bonding a plurality of tapes together in layers.
U.S. Pat. No. 3,741,796 to Walker is directed to the use of a plurality of torch flames, each resulting from the combustion of gaseous silicon tetrachloride and a mixture of hydrogen and oxygen directed upon a graphite mandrel to form a high-purity silica article upon the mandrel.
U.S. Pat. No. 3,840,350 to Tucker, Jr., is directed to a filament-reinforced composite metallic material which can be fabricated into various size filament-reinforced composite sheets or strips. A process is disclosed in which the metallic matrix of the composite consists of at least two plasma-sprayed particulated discrete metallic components which when subjected to a pressurized heat treatment will react to form a substantially homogenous alloy matrix for the filaments.
U.S. Pat. No. 3,888,661 to Levitt et al is directed to the preparation of a graphite fiber reinforced, metal matrix composite by hot-pressing. The composite comprises layers of a matrix metal selected from the group consisting of magnesium and magnesium based alloys in combination with alternate layers of a graphite fiber. Small additions of a metal selected from the group consisting of titanium, chromium, nickel, zirconium, hafnium, and silicon are made in order to promote wetting and bonding between the graphite fibers and the matrix metal.
U.S. Pat. No. 4,141,802 to Duparque et al is directed to an improvement in fabricating composite panels comprising a metal support foil to which a fiber-reinforced metal matrix layer adheres. The improvement is to interpose a thin layer of a bonding metal or alloy between the support foil and the fiber-reinforced metal matrix layer. The bonding metal layer serves to improve the adhesion of the metal matrix to the support foil and enables the metal matrix layer to be produced under less severe conditions.
U.S. Pat. No. 4,265,982 to McCreary et al is directed to a process of coating woven materials with metals or with pyrolytic carbon by chemical vapor deposition reactions using a fluidized bed. The porosity of the woven material is retained and the tiny filaments which make up the strands which are woven (including inner as well as outer filaments) are substantially uniformly coated.
U.S. Pat. No. 4,447,466 to Jackson et al is directed to a method of fabricating gas turbine engine, superalloy airfoils and other components by a method which uses low-pressure/high-velocity plasma spray-casting and segmented mandrels.
U.S. Pat. No. 4,594,106 to Tanaka et al is directed to flame spraying compositions exhibiting improved adherence to a variety of substrates, as well as articles coated with such compositions. The spraying compositions comprise a granulated mixture of two components: (1) a powdery material selected from the group consisting of powdered metals, heat resistant ceramics, cermets, and resins; and (2) a ceramic needle fiber such as whisker crystals of SiC or Si.sub.3 N.sub.4. Articles coated with thin films of these coatings exhibit thermal and corrosion resistance.
U.S. Pat. No. 4,595,637 to Eaton et al is directed to a process for plasma spraying small metal fibers onto the surface of a workpiece, and articles made using the process. An improved ceramic-faced metal article is made by spraying fibers onto the workpiece by injecting fibers into the plasma stream external to a plasma gun nozzle. Then, plasma sprayed ceramic particles are caused to surround the fibers as a matrix. Optionally a removable polymer material is interposed on the workpiece surface after the fibers are sprayed but before the ceramic matrix is sprayed to provide a low stiffness connector between a low thermal expansion coefficient ceramic material and a high expansion coefficient metal substrate. The connector alleviates strains from thermal expansion differences.
U.S. Pat. No. 4,627,896 to Nazmy et al is directed to a method of applying a corrosion protection layer to the base of a gas turbine blade by embedding particles of SiC in a metallic matrix by means of powder, paste or electrolytic/electrophoretic methods and compacting, welding, or fusing and bonding the matrix-forming material to the base by means of hot pressing, hot isostatic pressing or laser beam, electron beam, or electric arc.
None of the patents described briefly above discloses a method of manufacturing metal and ceramic composite materials utilizing thermal spray techniques which may include the formation of in-situ alloys and wherein the method may employ multiple torches, and which is applicable to continuous fiber type reinforcement structures as well as to discrete reinforcement materials which may be sprayed, including an optional finishing step of diffusion annealing and/or hot isostatic pressing.
The current trend in the technology of warfare is toward smarter, faster, and more maneuverable tactical guided missiles. A faster, more maneuverable tactical missile results in a combination of increased loads and heating on body structures and aerodynamic surfaces. The heating problem becomes increasingly severe as the missile velocity increases beyond Mach 6. The combination of increased loads and heating exacerbates an already difficult design problem, since most structural materials demonstrate decreasing strength and stiffness with increasing temperature. For example, Rene 41 is a commonly used high-strength high-temperature nickel base superalloy. Its specific strength and specific modulus at room temperature are 60.times.10.sup.4 inches and 1.1.times.10.sup.8 inches, respectively. Values for these properties drop to 40.times.10.sup.4 inches and 0.7.times.10.sup.8 inches at 1500 degrees F. for specific strength and specific modulus, respectively, and sharply accelerate downward with increasingly higher temperatures. Current materials are also deficient in one or more of the following attributes: cost, reliability, availability, and fabricability. There is a need for new fabrication methods which will produce metal and ceramic composite materials having greater specific strength and specific modulus at high temperatures and which can be manufactured at reasonable cost. Such composites should be substantially free of matrix-reinforcement interaction and degradation.